Using the pick block function (middle mouse click) while looking at a cauldron filled with a liquid does not do anything while It should select a cauldron.

To Reproduce:
1) Go into a creative world

2) Place down a cauldron and fill it with water/lava/power snow

3) Use the pick block function on the cauldron

4) observe that nothing happens while it should give you a cauldron or select it if you already have one